1. Field of the Invention
The invention relates to a drill, more particularly to the entering-end portion of a drill. Specifically the invention may be applied to a drill embodied in a self-drilling and self-tapping screw.
2. Description of the Prior Art:
Prior-art drills commonly have an entering-end or drill-point configuration which is chisel-shaped, i.e. has an end ridge transverse to the drill axis. The same is also true of prior-art self-drilling and self-tapping screws, for example as disclosed in U.S. Pat. No. 3,789,725. Any such chisel-end drill point will "walk" across the workpiece surface, by successive contacts alternately of the two ends of the ridge, if the drill is presented to the surface without rigid sideways support and other than aligned exactly perpendicular to the surface, which two problems are normally unavoidable in use of handheld tools. Furthermore, the chisel-end tends to rub the workpiece material rather than cut it.
Applicant is aware of proposals made in U.S. Pat. No. 3,079,831 and U.S. Pat. No. 3,937,120, to provide the entering end of a drill with two sharp points spaced apart transversely of the drill axis. However, both these proposals teach that the two points are level with each other along the drill axis and are equally spaced transversely from the drill axis. Consequently, if such a drill is presented to the workpiece surface without rigid sideways support and not aligned exactly perpendicular to the surface, the same problem of the drill entering-end "walking" sideways across the surface will still be encountered, due to successive contacts alternately of the two points with the workpiece surface. SUMMARY OF THE INVENTION
It is an object of the present invention to overcome this problem. The invention provides a drill comprising a shank rotatable about a longitudinal axis and including an entering end portion, the entering end portion comprising:
A SHARP POINTED TIP LYING SUBSTANTIALLY ON THE LONGITUDINAL AXIS OF THE SHANK;
A FIRST CUTTING EDGE INCLINED TO THE LONGITUDINAL AXIS OF THE SHANK AND EXTENDING FROM THE TIP AT THE LEADING END OF THE FIRST CUTTING EDGE TO THE PERIPHERY OF THE ENTERING END PORTION; A SECOND CUTTING EDGE INCLINED TO THE LONGITUDINAL AXIS OF THE SHANK SIMILARLY TO THE FIRST CUTTING EDGE AND ON THE SIDE THEREOF OPPOSITE TO THE FIRST CUTTING EDGE AND EXTENDING FROM A LEADING END TO THE PERIPHERY OF THE ENTERING END PORTION, THE LEADING END BEING OFFSET FROM THE TIP BOTH RADIALLY OUTWARDLY OF AND REARWARDLY LONGITUDINALLY ALONG THE SHANK AXIS, AND THE AXIAL EXTENT OF THE SECOND CUTTING EDGE BEING GREATER THAN ONE HALF OF THE AXIAL EXTENT OF THE FIRST CUTTING EDGE.
The sharp pointed tip presents a single point to the workpiece surface, on initial contact, which can dig into the surface without "walking". The first cutting edge starts cutting a hole which is sufficiently deep to stabilize the drill by the time the second cutting edge reaches the workpiece surface.